We interact almost effortlessly with objects in our three-dimensional (3D) visual environment, yet the image formed on the retina of each eye is simply a two-dimensional projection of 3D space that contains no explicit information about depth. Thus, a fundamental task of the visual system is to reconstruct 3D scene structure from the images formed on the retina of each eye. Quantitative information about depth is mainly carried by two visual cues: binocular disparity and motion parallax. The overall goal of this research is to understand where and how these depth cues are processed by neurons in the visual cortex to mediate our perception of a 3D world. This proposal describes experiments that address two fundamental issues regarding the neural basis of depth perception. In Aim #1, we will use a reversible inactivation technique to explore the roles that the dorsal and ventral visual processing streams make to stereoscopic depth perception. By inactivating areas MT and V4, we will test the hypothesis that the dorsal stream mainly processes absolute disparities to compute the location of objects in 3D space, whereas the ventral stream mainly processes relative disparities to compute 3D shape and fine depth structure. In Aim #2, we will carry out the first neurophysiological studies of how the visual system computes depth from motion parallax. Motion parallax resulting from movement of the observer is fundamentally ambiguous regarding the sign of depth (near versus far relative to the point of fixation). As a result, motion parallax generally must be combined with extraretinal signals to compute depth sign. We will test whether neurons in area MT combine retinal image motion with extraretinal signals to compute depth from motion parallax, and we will explore the origins of the extraretinal signals involved in this process. This research addresses the fundamental issue of how neural activity gives rise to visual perception, and also explores how extraretinal signals interact with visual processing to carry out interesting computations in the brain. Thus, this work addresses one of the Program Goals of the National Eye Institute's National Plan for Eye and Vision Research. The ultimate health-related value of this work will follow from a deeper understanding of how cognitive functions can be explained in terms of neural activity. Understanding the links between brain activity and mental function in normal observers will provide a deeper appreciation of the causes of various mental disorders.